9 Declarations [dcl.dcl]

9.11 Attributes [dcl.attr]

9.11.6 Fallthrough attribute [dcl.attr.fallthrough]

The attribute-token fallthrough may be applied to a null statement; such a statement is a fallthrough statement.
The attribute-token fallthrough shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
A fallthrough statement may only appear within an enclosing switch statement.
The next statement that would be executed after a fallthrough statement shall be a labeled statement whose label is a case label or default label for the same switch statement.
The program is ill-formed if there is no such statement.
The use of a fallthrough statement is intended to suppress a warning that an implementation might otherwise issue for a case or default label that is reachable from another case or default label along some path of execution.
Implementations should issue a warning if a fallthrough statement is not dynamically reachable.
end note
void f(int n) {
  void g(), h(), i();
  switch (n) {
  case 1:
  case 2:
  case 3:                       // warning on fallthrough discouraged
  case 4:                       // implementation may warn on fallthrough
    [[fallthrough]];            // ill-formed
end example